Plastic encapsulated semiconductor devices are susceptible to moisture ingress due to the permeable nature of plastic molding compounds. The amount of moisture that a plastic IC absorbs from its environment is dependent on several factors: the length of exposure time to the environment, the diffusivity of the plastic or how quickly moisture can be absorbed into the material, the solubility coefficient of the plastic or its saturation capacity, and the thickness of the plastic body on the IC. Devices containing moisture levels exceeding some critical amount run the risk of cracking or "popcorning" during the rapid heating of the solder reflow operation. Semiconductor devices which are subject to cracking are normally baked in an oven at approximately 125.degree. C., a typical temperature, for a predetermined length of time to drive moisture out of the devices before they are shipped to the customer. Those devices that are deemed to be moisture sensitive are packaged in "dry-packs" after baking to ensure that they are protected from moisture thereafter and will arrive dry at the customer site. Otherwise, devices that have absorbed a certain level of moisture run the risk of cracking during the solder reflow operation. Mechanical failure of the ICs often times lead to subsequent electrical failure of these same devices due to thermal and mechanical stresses induced on the ICs during their operation.
Current dry-packing practices involve baking semiconductor devices until dry, placing them into a dry-pack bag with desiccant packets and a humidity indicator card, sealing the bag immediately thereafter, and shipping the devices to the customer in these dry-packs. The amount of desiccant in each dry-pack bag can be determined by using IPC Standard SM786. A problem with the current method of dry-packing is that desiccant packets may not keep the dry-pack bag uniformly dry. Therefore, a single humidity indicator card is insufficient to give a true history of the conditions inside the dry-pack bag. The humidity indicator card is only accurate for a single site inside the bag. Furthermore, the location of the humidity indicator card relative to the desiccant also affects the amount of moisture that it senses, hence its accuracy. If the card is too close to the desiccant, then it will sense a drier atmosphere than if it were far away from the desiccant. It is impossible to accurately gauge how much moisture each IC has been exposed to.